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Notch Filter - AOE

Discussion in 'Electronic Components' started by ehsjr, Oct 19, 2006.

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  1. ehsjr

    ehsjr Guest

    According to Art of Electronics, there are prefab Twin-T based
    notch filters available from 1 Hz to 50 kHz. Anybody know who
    supplies/manufactures these?

    Thanks,
    Ed
     
  2. Joerg

    Joerg Guest

    Hello Ed,

    I doubt that those products would still be around. Maybe the odd one for
    50/60Hz or so. It's done digitally these days. Selecting capacitors to
    get to within a percent or less of tolerance is cost prohibitive these
    days. Switched capacitor filters are a modern option but even most of
    those chips are beginning to fade away.
     
  3. TI, UAF42 (no, not the vacuum tube UAF42!)

    http://focus.ti.com/docs/prod/folders/print/uaf42.html

    There is also a design software for free.


    Jorgen
    dj0ud
     
  4. Joerg

    Joerg Guest

    Hello Jorgen,
    Wow, didn't know they still made these. But Ed needs to hurry up,
    Digikey has a mere 5 left. $12.58 a pop.
     
  5. ehsjr

    ehsjr Guest

    That's a nice chip, & the software does the design work.
    I was hoping to find a drop in 3-legged device, from what
    I read in AOE. I hoped maybe they got around the precision
    cap problem by growing them on silicon.

    As you said, everything's gone active and moving toward
    DSP these days.

    Thanks to both you & Jorgen for your answers.

    Ed
     
  6. Joerg

    Joerg Guest

    Hello Ed,

    Caps on silicon are plagued by very large tolerances. Easily 30%. That
    would require expensive laser trimming. Another method would be
    sectioning and connecting islands by FET switches during final test and
    then storing the settings in flash. Both rather expensive propositions.

    What you can do on silicon is to build up several caps where the
    absolute values are not precise but where their ratios are very precise.
    That is what made switched capacitor filters popular which can also be
    nicely used for a notch filter. However, even those seem to have fallen
    from grace a bit given the small number of remaining offerings. My take
    is that they kept their prices too high, maybe trying to ride the "Oh
    look at this cool stuff" wave for too long. That's why I used them in
    only one design.
     
  7. Hal Murray

    Hal Murray Guest

    I wonder how much of that market area is now done in software?

    What was the upper limit on the useful frequency of switched cap
    filters?
     
  8. Joerg

    Joerg Guest

    Hello Hal,

    AFAIK the LTC1068 can be clocked to above 5MHz and if you'd design a
    filter with a clock/CF ratio of 25 you could manage 200kHz. Not
    impossible with a digital system but it does get expensive. However, the
    LTC1068 would also cost you over $5/1k.

    So nowadays it often looks like this: Audio is done in firmware while
    things above 100kHz are handled by ye olde LC circuitry, clever mixing
    and so on. Active filters have crept up into those regions as well but
    power consumption can become an issue.

    It all depends on what you want to do. There really is no way around the
    fact that discrete inductors show around 10% tolerance and capacitors
    with tolerances under 5% are expensive. Many times we analog guys first
    take a look around, see what's there in filters for consumer gear.
    Ceramic IF filters and so on. Then we try to come with a mixing scheme
    that lets us use those filters since the generation of carrier
    frequencies is inexpensive and precise.
     
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